The present invention relates generally to electromagnetic shielding, and more particularly to shielding electronic modules used in small, high density applications.
In a portable, two-way radio, modules provide a particular advantage in reducing the size of the unit. In order to minimize the space required, a number of components are placed in a module which typically contain a portion of the transmitter or receiver sections. A number of modules are situated on a main printed circuit (PC) board such that electrical connection between the modules is provided.
Each module generally includes a circuit board, which is enclosed in a housing having an open side to allow modules to be inserted. The connection between the module circuit board and the main printed circuit board is achieved by inserting contact pins from the module's circuit board into sockets located on the main PC board. The module's housing is generally made of conductive material, and forms part of the grounding circuit for the module.
It is known that external radio frequency interference (RFI) and/or electromagnetic interference (EMI) can degrade performance of electronic communication devices. RF interference becomes a problem especially in frequency synthesized communication equipment, wherein the synthesizer circuitry itself may become the source of interference. In order to reduce effect of the RFI, each module must be properly shielded. Accordingly, providing a grounded surface on the open side of the housing greatly improves the shielding. Presently a grounded, thin metal sheet is placed on the main PC board directly under the open side of the housing. The metal sheet connects at the periphery of the open side of the housing thereby electrically coupling the ground plane of the main PC board to the housing.
Another factor affecting performance of a frequency synthesized communication device is microphonics. Microphonics is a phenomena wherein mechanical movements of ground planes induce an audible tone in the speaker of the device. Since the housing constitutes part of the ground path of each module, any mechanism causing motion in the adjacent modules may produce microphonics. These movements can be invoked by compressing, squeezing or simply moving the device. Presently a number of insulators are placed between adjacent modules, thus separating the ground planes and minimizing microphonics.
Contemporary shielding and insulating methods offer some disadvantages since they require numerous undesired and difficult hand-placement operations in the manufacturing process. A high number of insulators and shielding materials is needed to produce the desired results. Additionally, a great degree of variability in shielding quality is observed.